Natural gas is the most important fuel gas in the United States and provides more than one-fifth of all the primary energy used in the United States. Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries. The composition of natural gas varies widely from field to field. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide or water vapor. On the other hand, streams that contain relatively large proportions of heavier hydrocarbons and/or other contaminants are common. Before the raw gas can be sent to the supply pipeline, it must usually be treated to remove at least one of these contaminants.
As it travels from the wellhead to the processing plant and ultimately to the supply pipeline, gas may pass through compressors or other field equipment. These units require power, and it is desirable to run them using gas engines fired by natural gas from the field. Since the gas has not yet been brought to specification, however, this practice may expose the engine to fuel that is of overly high Btu value, low octane number, or corrosive.
In the gas-processing plant itself, heavy hydrocarbons are often removed by condensation. Such a method is impractical in the field, however, because sources of external cooling or refrigeration are not available. Furthermore, cooling of the raw gas, which still contains substantial quantities of water vapor, is likely to bring the gas to a pressure/temperature/composition condition under which hydrates can begin to crystallize, thereby clogging the condensation equipment and preventing gas flow.
That membranes can separate C.sub.3+ hydrocarbons from gas mixtures, such as natural gas, is known, for example from U.S. Pat. Nos. 4,857,078, 5,281,255 and 5,501,722. Separation of acid gases from other gases is taught, for example, in U.S. Pat. No. 4,963,165. It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
It is also known to operate membrane systems at reduced temperatures by deliberately cooling the incoming feed stream, as is taught in U.S. Pat. No. 5,352,272, and to use the Joule-Thomson cooling produced by transmembrane permeation to facilitate upstream condensation, as is taught in U.S. Pat. No. 5,762,685.
The problem of upgrading raw gas in the field, such as to sweeten sour gas, is addressed specifically in U.S. Pat. No. 4,370,150, to Fenstermaker. In this patent, Fenstermaker teaches a process that uses a membrane, selective for hydrogen sulfide and/or heavier hydrocarbons over methane, to treat a side stream of raw gas. The process produces a membrane residue stream of quality appropriate for engine fuel. The contaminants pass preferentially through the membrane to form a low-pressure permeate stream, which is returned to the main gas line upstream of the field compressor. Such a process relies on there being sufficient compressor capacity available to handle the return stream that is recycled to the compressor inlet. However, if the raw gas requires more than a minor adjustment in composition, the proportion of gas that has to be recycled to the compressor may be comparatively large. For example, to upgrade the methane content from 70% to 80%, or from 80% to 90%, may require as much as 50% of the gas being treated by the membrane to be returned for recompression. If the gas is more heavily contaminated, such as containing hydrogen sulfide at the percent level, for example, as is not uncommon, the proportion returned on the low pressure side may be even higher, such as 60% or more. As well as diverting compressor capacity, this makes for an inefficient use of fuel, since fuel gas created by the membrane is used in part to recompress the fuel reject stream.
There remains a need for a process that provides an acceptable fuel gas for field use, but that is more efficient in terms of fuel use and recompression requirements.